Method for protecting the diaphragm and extending the life of SiC and/or Si MEMS microvalves

ABSTRACT

A microvalve and a method of forming a diaphragm stop for a microvalve. The microvalve includes a first layer and a diaphragm member to control the flow of fluid through the microvalve. The method comprises the step of forming a contoured shaped recess extending inward from a surface of the layer by using a laser to remove material in a series of areas, at successively greater depths extending inward from said surface. Preferably, the recess has a dome shape, and may be formed by a direct-write laser operated via a computer aided drawing program running on a computer. For example, CAD artwork files, comprising a set of concentric polygons approximating circles, may be generated to create the dome structure. The laser ablation depth can be controlled by modifying the offset step distance of the polygons and equating certain line widths to an equivalent laser tool definition. Preferably, the laser tool definition is combined with the CAD artwork, which defines a laser path such that the resulting geometry has no sharp edges that could cause the diaphragm of the valve to tear or rupture.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to methods for fabricating microvalves.More specifically, the invention relates to procedures for forming acontoured valve stop in a microvalve.

2. Background Art

Microvalves may be used to control the flow of a gas in hightemperature, high corrosive environments. In one such microvalve, a thindiaphragm, sandwiched between adjacent SiC or Si layers or wafers, isused to control the flow of gas through the valve. In use, the diaphragmabuts against a seat around an inlet of the valve to close that inlet;and when the pressure of gas in this inlet rises above a given value,the diaphragm flexes upwards, allowing gas flow through the inlet andthrough the valve.

A cavity or recess is provided inside the valve to allow this upwardflexing movement of the diaphragm; and as the diaphragm flexes upward,the diaphragm comes into contact with surfaces of this cavity. Thiscontact may cause significant localized forces or stresses on thediaphragm, or specific portions of the diaphragm.

Shaping this cavity or recess in any particular way is difficult becauseof the very small dimensions involved. For instance, this cavity mayhave a width or diameter of about one millimeter and a depth of about0.025 millimeters. In the past, reactive ion etch (RIE) has been used tocreate very small, three-dimensional features in SiC and/or Si wafers.RIE etches can provide excellent trenches with steep smooth side walls,but RIE does not lend itself to creating contoured shapes. Since RIEuses a metal mask, it precludes using a gray scale mask. In addition,RIE is time consuming (etch rate of 0.001 mm/min). Inductively coupledplasma (ICP) etch techniques can also make clean well-defined trenches,but these techniques also are not conducive to making sloped andcontoured surfaces. Lastly, the microvalve features are much too smallfor mechanical machining.

SUMMARY OF THE INVENTION

An object of this invention is to provide an improved microvalve.

Another object of the present invention is to extend the life of amicroelectromechanical structure (MEMS) microvalve diaphragm.

A further object of the invention is to provide a dome-shaped contouredvalve stop, in a microvalve, to reduce stress concentrations at the edgeof a thin diaphragm of the valve.

Another object of the invention is to form a dome-shaped contoureddiaphragm stop in a microvalve, by combining a laser tool definitionwith CAD artwork which defines a laser path such that the resultinggeometry has no sharp edges that would cause the diaphragm to tear orrupture.

These and other objectives are attained with a microvalve and a methodof forming a diaphragm stop for a microvalve. The microvalve includes afirst layer and a diaphragm member to control the flow of fluid throughthe microvalve. The method comprises the step of forming a contouredshaped recess extending inward from a surface of the layer by using alaser to remove material in a series of areas, at successively greaterdepths extending inward from said surface. Preferably, the recess has adome shape, and may be formed by a direct-write laser operated via acomputer aided drawing program running on a computer.

For example, CAD artwork files, comprising a set of concentric polygonsapproximating circles, may be generated to create the dome structure.The laser ablation depth can be controlled by modifying the offset stepdistance of the polygons and equating certain line widths to anequivalent laser tool definition. Preferably, the laser tool definitionis combined with the CAD artwork, which defines a laser path such thatthe resulting geometry has no sharp edges that could cause the diaphragmof the valve to tear or rupture.

Further benefits and advantages of the invention will become apparentfrom a consideration of the following detailed description, given withreference to the accompanying drawings, which specify and show preferredembodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a microvalve embodying the presentinvention.

FIG. 2 is a partial cut away perspective view of the microvalve.

FIG. 3 is a cross-sectional view of the microvalve.

FIG. 4 is a top view of the bottom layer of the microvalve.

FIG. 5 illustrates a procedure for forming a valve stop in themicrovalve.

FIG. 6 shows the interior, dome pattern of the valve stop.

FIG. 7 is an enlarged view of a portion of FIG. 6.

FIG. 8 illustrates an alternate dome pattern.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1-4 show a microvalve 10 generally comprising first and secondlayers 12 and 14, diaphragm member 16, and switching means 20. Switchingmeans 20, in turn, includes boss 22 and actuation mechanism 24.

Generally, each of the layers 12 and 14 has a flat thin shape, and thelayers are secured together to form a plate or valve body 26. This valvebody forms an inlet opening 30 for receiving fluid, an outlet opening 32for conducting fluid from the valve body, and a flow channel 34 forconducting fluid from the inlet to the outlet. The diaphragm 16 isdisposed between the layers 12 and 14, and is moveable between open andclosed positions. In the closed position, the diaphragm 16 blocks theflow of fluid from the inlet 30 to the flow channel 34; and in the openposition, the diaphragm allows fluid flow from the inlet into that flowchannel, allowing fluid flow from the inlet to the outlet 32 of themicrovalve. The diaphragm 16 is biased to the closed position, and movesfrom the closed position to the open position when the pressure of fluidin the inlet 30 reaches a preset value. The switching means 20 isconnected to the valve body 26 for moving the diaphragm 16 to the closedposition against the pressure of fluid in the inlet 30.

In the preferred embodiment, the two layers 12 and 14 are each amaterial such as Silicon Carbide (SiC), Silicon (Si), or cermaic and arebonded together with a thin metallic diaphragm 16. The bottom layer 12has through holes that form the valve inlet 30 and outlet 32, and layer12 forms the flow channels 34 for the fluid. Also, preferably, asmentioned above, the switching means 20 includes a boss 22 and anactuation mechanism 24. The top layer 14 contains the boss 22, which isadjacent to the diaphragm 16, and the actuation mechanism 24 is used topush the boss, which in turn pushes the diaphragm against a seat 30 aextending around the inlet 30 and thereby to close the valve 10.

With the embodiment of the microvalve 10 shown in FIGS. 1-4, theactuation mechanism 24 is a shape memory alloy, such as atitanium-nickel alloy. Other actuation mechanisms may be used such aspiezoelectric ceramics, electromagnetics or microsolenoids.

In use, when a high gas pressure is supplied to the bottom feed throughhole 30, the diaphragm 16 and boss 22 are pushed away from the seat 30a. If a sufficient enough high pressure is supplied, the diaphragm 16 ismoved away from the seat 30 a, allowing the gas to flow over and aroundthis seat. The gas then flows through channels 34, which preferably areetched in the bottom SiC or Si layer 12, and exits through anotherthrough hole 32. The pressure at which gas will flow can be chosen bysuitable choices of the material and thickness for diaphragm 16 as wellas the actuation mechanism 24. For instance, with the embodiment of themicrovalve 10 illustrated in the drawings, the pressure at which the gaswill flow, referred to as the cracking pressure, is approximately 800psi. Other pressures, or pressure ranges, may be used, however, and forexample, the valve 10 can be modified to open at pressures from 800-1200psi. These pressures, it may be noted, make the valve well suited forhigh pressure gas operation.

When the actuation mechanism 24 is activated, the boss 22 is pusheddown, closing the diaphragm 16 against the boss seat 30 a and stoppingthe flow of gas. With the embodiment of microvalve 10 shown in FIGS.1-4, in which the actuation mechanism 24 is a shape memory alloy, theactuator is activated by conducting an electric current through thealloy, resistively heating the material and causing a change in thematerial modulus. The shape memory bars, which are in contact with theboss 22, effectively become stiffer, pushing the boss back down againstthe boss seat 30 a. As mentioned above, other types of actuators, suchas piezoelectric ceramics, microsolenoids, and electromagnets, may alsobe used in microvalve 10. Due to the design and construction, theactuation mechanism 24 is preferably located on the top surface of thetop SiC or Si layer 14. This allows for easy implementation of a widevariety of actuation mechanisms 24. This location of the actuationmechanism 24 also helps to isolate that mechanism from the hightemperature gas with which the valve 10 may be used.

A recess or cavity 50 in the valve body 26 is provided to allow thediaphragm 16, specifically the central portion thereof, to flex upward,away from valve seat 30 a. With the preferred embodiment of microvalve10, a sloped “dome” is ablated in the underside of the top SiC or Siwafer in order to provide a gentle valve stop. This extends valve lifeby reducing stress concentrations on the valve diaphragm 16. The smoothsloped edges of the recess provide a gentle stop and prevent rupturingthe thin diaphragm of the valve. With reference to FIG. 5, adirect-write laser 52, controlled via a computer aided drawing programrunning on computer 54, may be used to form the desiredthree-dimensional contoured shapes in silicon carbide and siliconwafers. These materials absorb the laser energy sufficiently enough tobe ablated in a relatively short period of time (it takes only a fewminutes to pattern an entire wafer). Laser 52, for example, may be ofthe type that are currently designed for drilling holes through variousdielectrics or conductors by ablating away material.

By adjusting several laser parameter settings, and applying theappropriate CAD input files, contoured shapes can be formed in thewafers. One suitable laser that may be used, for instance, is a tripledYAG laser having a nominal spot size of twenty-five microns, a Gaussianpower distribution, and 355 nm wavelength. In this mode of operation thelaser action is similar to a pen plotter and multiple tools can bedefined for the laser in the same way that multiple tools are definedfor photoplotters. The repetition rate controls the amount of power(energy per pulse) delivered by the laser. Furthermore, parameters suchas settling time, drill style, defocus, etc., all combine to define alaser “tool.” Multiple tools can be defined for the laser in the sameway that multiple tools are defined for photoplotters. By experimentingwith different wafer materials, laser tool definitions, and CADrepresentations, a wide range of suitable three-dimensional contours canbe achieved. Other types of lasers, such as 248 nm Excimer, could alsobe used with the appropriate settings.

For example, CAD artwork files, comprising a set of concentric polygonsapproximating circles, may be generated to create the dome structure.The laser ablation depth can be controlled by modifying the offset steppitch of the polygons and equating certain line widths to an equivalentlaser tool definition. Preferably, the laser tool definition is combinedwith the CAD artwork, which defines a laser path such that the resultinggeometry has no sharp edges that could cause the diaphragm to tear orrupture.

FIGS. 6-8 illustrate, as examples, specific patterns that may be used toform the dome-shaped recess. The pattern 60 shown in FIGS. 6 and 7 iscomprised of a series of polygons 62. In this pattern, the spacingbetween lines is approximately constant, and the widths of the linesvary between 1 and 5 um. Other arrangements may be used, however, andfor example, the pattern could be formed with approximately constantline widths, and the spacing between lines could vary. FIG. 8illustrates an alternate pattern 70 comprised of one hundred thirty-twoconcentric, thirty-six sided polygons. These polygons are separated intoseven radially adjacent groups 70 a, 70 b, 70 c, 70 d, 70 e, 70 f and 70g. The polygons in each group are equally spaced apart, and the spacingbetween adjacent polygons increases from group to group in the radiallyoutward direction

The above-described process for forming dome shaped recess 50 is easilycontrolled, repeatable, and much faster than etching approaches. Theprocess is capable of creating features and shapes which cannot becreated by standard reactive ion etching (RIE) processing. Also, theapproach described above does not require creation of a hard photomaskor gray scale mask.

Known MEMS fabrication processes may be used for the construction of thewhole valve 10. This allows the possibility for low cost microvalves dueto the economies of scale—many valves can be fabricated per substratewafer. In addition, due to the inherent accuracy of these fabricationprocesses, such as lithography, microvalves can be produced with veryprecise features. In addition, preferably, the diaphragm material andthickness (choosing modulus and thickness to determine membrane“stiffness”) and the actuation mechanism are chosen such that themicrovalve can control very high pressure gas flows. The approximatepressure range of the current preferred embodiment is between 800-1200psi.

As will be understood by those of ordinary skill in the art, theindividual elements of microvalve 10 may have a wide range of specificdimensions. With one specific embodiment, for example, that has beenactually reduced to practice, the shape memory alloy TiNi bars 24 are0.010″ square and 0.040″ long, the diaphragm 16 is 0.0005″ thick, andthe boss 22 has a diameter of 0.020″. Also, with this embodiment, theflow inlet 30 and the flow outlet 32 each have a diameter of 0.020″, andthe flow channel 34 is 0.002″ deep. In addition, the dome shaped recess50 has a diameter of 1.270 mm, the outer edge of the dome is flush withthe wafer surface and the center of the dome is 0.025 mm high. Thisallows the valve diaphragm 16 to open and still provide a smooth surfaceto stop the back of the diaphragm and prevent rupturing up to at least1200 psi pressure.

As will be understood, valve 10 may be changed in many ways withoutdeparting from the scope of the present invention. For example, withmodifications within the ability of those of ordinary skill in the art,the functions of inlet 30 and outlet 32 may be reversed, so that fluidflows into valve 10 via opening 32 and exits the valve via opening 30.Also, any suitable material or materials may be used to form layers 12and 14. As mentioned above, these layers 12 and 14 may be made from aceramic, and these layers may also be made from a metal alloy such asstainless steel.

The preferred embodiment of the invention, as described above in detail,provides a number of important advantages. For instance, microvalve 10is very well suited for use in applications where high temperatures orcorrosive conditions exist. SiC is a very robust material againstchemical corrosion. At the present time, there is no known chemical thatwill etch SiC at a rate greater than 100 Angstroms/min. Therefore, amicrovalve constructed or manufactured from SiC will resist potentiallycorrosive chemicals in the environment. Further, microvalve 10 cancontrol a very high pressure gas feed due to the mechanical design aswell as the force capabilities of the actuation mechanism. Thecombination of robustness against chemical corrosion, high temperaturecompatibility, and high pressure control makes microvalve 10 ideallysuited for harsh environments. These may include such applications asaircraft engines, power turbines, or missile systems.

While it is apparent that the invention herein disclosed is wellcalculated to fulfill the objects stated above, it will be appreciatedthat numerous modifications and embodiments may be devised by thoseskilled in the art, and it is intended that the appended claims coverall such modifications and embodiments as fall within the true spiritand scope of the present invention.

What is claimed is:
 1. A method of forming a diaphragm stop for amicrovalve, the microvalve including a first layer and a diaphragmmember to control the flow of fluid through the microvalve, the methodcomprising the step of: forming a contoured shaped recess extendinginward from a surface of the layer, including the step of using a laserto remove material in a series of areas, at successively greater depthsextending inward from said surface.
 2. A method according to claim 1,wherein the step of using a laser includes the step of using the laserto remove mateial in a series of generally concentric areas, atsuccessively greater depths extending inward from said surfaces.
 3. Amethod according to claim 1, wherein the forming step includes the stepof forming a smooth contoured shaped recess extending inward from thesurface of the layer.
 4. A method according to claim 2, wherein adjacentconcentric areas are spaced apart between 1 and 10 um.
 5. A methodaccording to claim 1, wherein each of said areas is polygon shaped.
 6. Amethod according to claim 5, wherein each of said areas is a polygonhaving at least thirty sides.
 7. A method according to claim 1, whereineach of said areas is circular.
 8. A method according to claim 2,wherein said series of concentric areas are uniformly spaced apart.
 9. Amethod according to claim 1, wherein the step of using a laser includesthe steps of: using a computer to control movement of the laser; andprogramming the computer to control the laser to ablate a series ofconcentric, polygon shaped areas from the layer, said series extendinginward from the surface of the layer.
 10. A method according to claim 1,wherein in the microvalve, the diaphragm member includes a centralportion that moves between open and closed positions, and in the openposition, the central portion of the diaphragm has a given shape, andthe step of forming the recess includes the step of forming the recesswith a shape that substantially matches said given shape.
 11. A methodaccording to claim 1, wherein the diaphragm member has a thickness ofsubstantially 0.0125 mm and, the recess has a diameter of substantially1.270 mm and a depth of substantially 0.025 mm.